Data communication on the land mobile satellite channel using small, low-gain antennas presents challenges in the design of digital modems. Demodulation of signals with marginal carrier-to-noise ratios requires that processing techniques achieve the highest possible efficiency. Frequency variations due to Doppler shift and oscillator drift can be a significant fraction of the symbol rate and must be tracked to minimize detection losses. Also, maintaining symbol synchronization is critical in the presence of multipath fading and shadowing experienced in the land mobile satellite environment.
An all digital demodulator using off-the-shelf VLSI components is known in the prior art. In particular, a multirate binary phase shift keying (BPSK) demodulator using the digital VLSI implementation is disclosed in J. F. Roesch, Jr., "An All Digital Demodulator Using Off-the-Shelf VLSI Technology," Milcom. Trans., Vol. 3, pp. 46.4.1-46.4.5, October 1986, which is incorporated herein by reference.
Power efficiency is a major consideration in the land mobile satellite system because a low-gain, omnidirectional antenna is a requirement. However, an omnidirectional antenna will not effectively discriminate between direct-path signal energy and reflected signal energy. With the effects of Doppler shift, these multipath signals can cause random variations in the signal amplitude and phase. In the land mobile satellite channel, multipath fading may be too severe to allow effective estimation of the carrier phase for coherent demodulation. Therefore, there is a need for a digital modem that achieves symbol synchronization independently of carrier phase or frequency at low signal-to-noise ratios. In addition, the digital modem should improve acquisition speed and reliability relative to the traditional square-law nonlinearity of a bandpass filter approach.